Inverters are known generally and are devices which transform DC (direct current) electrical energy, such as from a fuel cell or the like, into AC (alternating current) electrical energy suitable for use by utility companies or other consumers of electrical energy. Most inverters include at least one pair of switching elements, and by alternatively actuating each switch, electrical energy from the DC source flows through the load first in one direction and then in the reverse direction forming a fundamental AC waveform.
Numerous different types of switching devices can be employed in an inverter to reverse the current through the load. Semiconductor switches, such as thyristors, are frequently used in present day inverters and this type of device is typically unidirectional so that the high energy current pulses pass therethrough in one direction when turned on by a suitable signal applied to its control terminal. Many thyristors, as is known, will not change from a conducting state to a nonconducting state merely upon the removal of the control signal from the control terminal but require that the instantaneous magnitude of current passing therethrough to be reduced to zero together with a small positive bias before transitioning to the nonconducting state. The process by which the current is redued to zero through the thyristors under load so that transition to the nonconducting state can occur is known as "commutation" and numerous circuit configurations have been proposed for this purpose. Many commutation circuits operate by presenting a commutation pulse to the load from a storage device, such as a capacitor or resonant circuit, for a period of greater than the turn-off time of the thyristor. Since during this period the load current is supplied by the storage device, the instantaneous magnitude of the current through the thyristor drops to zero for a sufficient period to allow transition to the nonconducting state.
Of interest is U.S. Pat. No. 4,204,268 by J. R. Vivirito issued on May 20, 1980 for AUXILIARY COMMUTATION CIRCUIT FOR AN INVERTER, assigned to the same assignee as the present invention. The disclosed apparatus includes an auxiliary commutation circuit of the impulse commutated bridge inverter type in which additional commutation energy is stored on a pair of oppositely charged capacitors. Rather than operate on every cycle, the commutation circuit includes switches in series with the charge capacitors that are operated only in response to a sensed overcurrent condition to provide the additional stored energy required for commutation of the main thyristors.
Another disclosure of interest in U.S. Pat. No. 4,225,912 to G. J. Messer, issued on Sept. 30, 1980 for CONTROL FOR AN AUXILIARY COMMUTATION CIRCUIT, also assigned to the same assignee as the present invention. A control circuit used with an auxiliary commutation circuit of an inverter responds to the increased time period resulting from the additional capacitance of the auxiliary capacitors to extend the initiation of the makeup pulse. This allows a natural decay of the commutation pulse so that the makeup pulse begins when the instantaneous value of the current through the thyristor is low.